(a) Field of the Invention
The present invention relates to an adaptive array antenna system for a mobile communication base station. More specifically, the present invention relates to a transmitting and receiving apparatus and method in an adaptive array antenna system capable of real-time RF calibration required when a mobile communication base station system forms a digital beam.
(b) Description of the Related Art
In general, an adaptive array antenna system that adaptively directs an antenna beam in a desired direction using an array antenna in a mobile communication system can increase an antenna gain to improve a signal-to-noise ratio. In the case where a plurality of users exist in the same band, for example, in the case of a mobile communication base station system, the users can form independent antenna beams to reduce interference among the users by employing a smart antenna system.
The smart antenna system forms an antenna beam in an RF band using an analog device, or converts an RF signal to a baseband signal and then forms a digital antenna beam. The method of forming the digital antenna beam is more popular than the analog method because it can easily generate a beam pattern.
The digital method down-converts an RF signal received through an array antenna to a digital signal using an independent receiver, and then changes the amplitude and phase of the digital signal to form a digital beam. Accordingly, in order to accurately control a direction in the baseband, a relative amplitude and phase of the RF signal in an array antenna stage must be maintained in the baseband. However, an actual array receiving system has different amplitude and phase transfer functions for each array channel because of analog device characteristics comprising the receiver and other causes. Thus, a function of periodically measuring and compensating the different transfer functions is required, which is called RF calibration for an array receiver. The same concept is applied to RF calibration for an array transmitter.
The RF calibration requires a process for estimating a transfer function of an array receiver. The transfer function of the array receiver is varied depending on a state of an automatic gain controller as well as surrounding environments such as temperature, humidity, and so on. Thus, the transfer function should be periodically measured and calibrated when the array receiver is operated, which is called real-time calibration. While the temperature and humidity are slowly varied with time, a gain of the array receiver is determined by a time constant of the automatic gain controller. Accordingly, the transfer function must be estimated at a small time interval in order to track the receiver gain in real time.
The transfer function of the receiver is estimated by analyzing the calibration signal that has simultaneously passed through a plurality of receivers to find a gain and phase variation of the array receiver. In the real-time calibration, the signals received through an antenna act as interference to the calibration, the effect of which should be removed during the calibration. Though the effect of received signals can be reduced when the power of the calibration signal is increased, the system performance will be deteriorated because the calibration signal is interference to the antenna signals. Accordingly, a matched filtering based calibration method with a long integration time and a small calibration signal power is frequently used. However, the matched filtering requires exact timing information and the multiplication operations to result in complicated hardware.